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Astronomers Discover Cold, Warm and Hot Gas Around a Young Brown Dwarf

11.01.2005

A team of scientists from the University of Delaware has discovered that brown dwarfs--celestial bodies that are often referred to as failed stars--can be surrounded by clouds of very hot and very cool gas.

The UD research team of John E. Gizis, assistant professor of physics and astronomy, Harry L. Shipman, Annie Jump Cannon Chair of Physics and Astronomy, and James A. “Rusty” Harvin, researcher in physics and astronomy, used the Hubble Space Telescope to show for the first time that a brown dwarf was associated with a cloud or disk containing molecular hydrogen gas.

The team members presented their results in a poster session dedicated to the study of brown dwarfs and exoplanets and at a news conference held Monday, Jan. 10, during a meeting of the American Astronomical Society in San Diego, Calif.

The UD researchers studied the space object that goes by the name of 2MASSW J1207334-393254, or simply 1207, for the past year. The object is located in the constellation of the Centaur but is so small and dim that it is impossible to see with the unaided eye or even with a pair of binoculars.

The team obtained ultraviolet light observations from a piece of equipment on the Hubble Space Telescope known as the Space Telescope Imaging Spectrograph, or STIS, which failed just weeks after the observations were made.

The team’s most interesting finding was that this brown dwarf was surrounded by a cloud of cold gas containing the common element hydrogen in molecular form. The fingerprints of molecular hydrogen are concentrations of ultraviolet emissions at particular wavelengths, in this case 1503 and 1530 Angstroms.

Hydrogen molecules had previously been found around very young stars but never before in a brown dwarf. "It’s really amazing that the gas around this tiny brown dwarf is behaving so much like the gas around much more massive newly forming stars, called T Tau stars," Gizis said.

Also noteworthy was the finding of other ultraviolet fingerprints from very hot gas. "This star, in the ultraviolet, looks much like bigger and hotter stars like our sun," Shipman said. “Our own sun is surrounded by a layer of warm gas, called the chromosphere, and a layer of very hot gas, called the corona. A thin layer of hot gas, with temperatures of a hundred thousand degrees, separates the two. The brown dwarf 1207 has warm gas and the hot hundred thousand-degree gas. So far, no one has detected the very hot gas in any possible corona around 1207.”

An interesting characteristic of the spectrum is the absence of the chemical element silicon. Most stars that show evidence of hot gas also show signs of familiar chemical elements like carbon, nitrogen and oxygen, all of which are seen in the ultraviolet light from 1207. But, these other stars also show silicon, which Shipman said is absent in the ultraviolet light from 1207. "A likely place for the silicon is in a dusty disk," Shipman said, because the dust would contain a lot of silicon, just like rocks on Earth.

The team would like to look for similar clouds of hot and cool gas around other brown dwarfs, but it will have to wait for successful STIS repair missions. The proposed repair missions would include installation of the Cosmic Origins Spectrograph, or COS, on the Hubble, which could do the additional observations.

"We had this door opened, showing us some wonderful things happening around a young brown dwarf, but then with the failure of STIS and the uncertainty about repair missions, the door was closed again," Gizis said.

Brown dwarfs are larger than planets like Jupiter and smaller than stars. Stars, which are at least 70 times as massive as Jupiter, shine because of self-sustaining nuclear reactions in their cores. When gravity forces young brown dwarfs like 1207 to contract, their insides heat up and they glow, allowing observers to detect them.

The visible light from 1207 indicates that it is fairly hot for a brown dwarf, with a temperature around 2500 degrees, Shipman said. Its presence in a young star-forming region indicates that it is only about 10 million years old, a newborn baby in astronomical terms.

The UD team’s work is supported by the National Aeronautics and Space Administration (NASA) and by the National Science Foundation.

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